Nanoscale gold particles exhibit physical and chemical properties different from those of ordinary coarse gold grains. In particular, nanoscale gold is catalytically active and can be used as a catalyst for oxidizing carbon monoxide to form carbon dioxide. Catalytically active gold also has been proposed for use in catalyzing other oxidation reactions, such as the oxidation of carbonaceous soot in diesel exhaust streams, the oxidation of unsaturated and saturated hydrocarbons, and the like.
Generally, nanoparticles of gold are very mobile, possess large surface energies, and tend to coalesce easily. Coalescence has been difficult to prevent, making gold difficult to maintain in nanoparticle form. Such coalescence is undesirable, as the catalytic activity of gold tends to fall off as its particle size increases. This problem is relatively unique to gold and is much less of an issue with other noble metals such as platinum and palladium. Thus, methods to deposit and immobilize gold nanoparticles on a carrier in a uniformly dispersed state have been sought.
The primary methods developed to date to deposit catalytically active gold on various supports include (i) coprecipitation, in which a support and gold precursors are brought out of solution as hydroxides, for example, by adding a base such as sodium carbonate; (ii) deposition-precipitation, in which a gold precursor is precipitated onto a suspension of pre-formed support by an increase in pH; and (iii) a method in which a gold-phosphine complex (for example, [Au(PPh3)]NO3) is made to react with a freshly precipitated support precursor. Other procedures such as the use of colloids, grafting, and vapor deposition have met with varying degrees of success.
The above-cited methods, however, have suffered from serious reproducibility problems. Such reproducibility problems have resulted from difficulty in controlling gold particle size, poisoning of the gold catalyst by ions such as chloride, loss of active gold in pores of the support, necessity in some cases of thermal treatment to activate the gold catalyst, inactivation of certain catalytic sites by thermal treatment, lack of control of gold oxidation state, and the inhomogeneous nature of the hydrolysis of gold solutions by the addition of aqueous base.
Gold catalysts produced by the above-mentioned deposition-precipitation method have typically been activated by thermal treatment in air. In at least one case, however, such thermal treatment has been carried out in an atmosphere comprising ozone. The latter process yielded a more stable but less active catalyst than a corresponding catalyst that was activated in air.
Physical vapor deposition (PVD) techniques have been used to deposit gold on various non-nanoporous support media, such as ceramic titanates that were made under conditions so as to lack nanoporosity. More recently, effective heterogeneous catalyst systems have been produced by providing catalytically active gold on nanoporous supports including a composite support derived from relatively small titania particles (referred to as guest material) that at least partially coat the surfaces of larger alumina or activated carbon particles (referred to as host material). These composite systems have provided effective catalytic performance with respect to carbon monoxide oxidation.
In short, gold offers tremendous potential as a catalyst, but the difficulties involved in producing and maintaining gold in nanoparticle form have continued to hinder the development of commercially feasible, gold-based catalyst systems.